Those skilled in the art know optical devices for detecting the reference angular positions of wheels, in particular coaxial wheels belonging to the gear train of a clockwork movement. All these optical detection devices are relatively complex. They require a light source and a light receiver at a distance from said source. The mounting of an optical detection device is often difficult given the precise adjustment necessary between the light source, the receiver and the wheels associated with such device. In the case of the present invention, one thus proposes omitting any optical wheel position detection system for a device of small dimensions.
International Patent Application No. 97/45705 discloses a system for detecting the position of coaxial wheels respectively associated with coaxial hands of a clockwork movement. On each of the coaxial wheels, formed of full plates, are arranged several discrete portions of a hard magnetic film defining an identification pattern for a contactless inductive sensor. An inductive sensor is associated with each of the coaxial wheels. In other words, there are as many magnetic inductive sensors as coaxial wheels whose angular positions have to be detected, in particular given reference positions. The amplitude of the signal detected by the inductive sensors varies according to whether or not they are situated in front of a segment of magnetic material, as is shown in particular in FIGS. 3 and 4 of the International Patent Application.
The detection system proposed in Patent Document No. WO 97/45705 is disadvantageous in that each wheel associated with an inductive sensor must be fitted with the segment or distinct portion of a solid magnetic film arranged on one face of the plate of the wheel. The manufacturing cost of the wheels is thus increased and the wheel plates are full or solid, which can be a drawback for the working of the clockwork movement, in particular from the point of view of power consumption. The wheel plates are formed of at least two different materials, namely the material forming the plate itself and the magnetic material arranged on one surface of this plate. In addition to these drawbacks, a major drawback relating to the cost of the detection system proposed and to the space requirement thereof will be mentioned, namely that there are as many inductive sensors as wheels whose position has to be detected. Thus, for three coaxial wheels, there are three inductive sensors respectively associated with said three wheels.